The present invention relates to a semiconductor device including memory cells having floating gates and more particularly relates to a semiconductor device such as "One Time Programmable read only memory." (hereinafter abbreviated to "OTP")
An OTP has memory cells of the same configuration as memory cells for Erasable Programmable Read Only Memory (hereinafter abbreviated to "EPROM"), i.e. Programmable Read Only Memory (P-ROM) equipped with memory cells constructed using Floating gate Avalanche injection MOS (hereinafter abbreviated to "FAMOS") transistors. An EPROM is provided with an ultraviolet irradiation window for use in erasing data but the OTP is not.
A user can therefore write data once to the OTP but once written, the data cannot be erased. That is, data cannot be written for an evaluation test after an OTP memory chip was packaged. Because of this, conventionally, data has been read from the OTP memory cells with nothing written therein (at this time, all of the data should be "1") and it has been recognized that there are no structural failure when all of the read data are "1".
However, the above test method has a drawback that structural failure cannot be detected when there is a short between adjacent word lines or between adjacent bit lines. As a result, conventionally, it has not been possible to obtain highly precise results in the evaluation test after the OTP chip was packaged. For example, even if an electrical short have occurred between word lines or bit lines due to thermal stress occurring in the packaging process, it has not been possible to detect such short. This has presented a problem with regards to reducing shipments of defective OTP chips.
When one of the data read in the evaluation test after the OTP chip was packaged is "0", an OTP deficiency can be detected. However, in either case, it is not possible to know whether the cause is a short between word lines, a short between bit lines, or some other cause. This has made it difficult to carry out a test capable of reducing the occurrence of deficient products.